Sirajganj: Soil sample was collected from agricultural field of Sirajganj. The GPS location of the sampling point where soil samples were collected was approximately 24˚24'27.7164''N and 89˚38'7.1376''E. Sirajganj falls under Precambrian platform and is characterized by sedimentary rocks above a Precambrian igneous and metamorphic basement. The area lies in the active delta region of

Brahmmaputra-Jamuna floodplain and general soil type is grey floodplain soils (Figure 2). The climate is characterized by high temperature, heavy rainfall, often excessive humidity and fairly marked seasonal variation.

Tangail: Soil sample was collected from agricultural field which is situated beside Joydevpur-Tangail-Jamalpur highway. The GPS location of the sampling point where soil samples were collected was approximately 24˚19'35.1336''N and 89˚55'33.7656''E. The parent material of soil is Brahmaputra-Jamuna alluvium and general soil type is grey floodplain soils (Figure 2). The area is covered by the recent alluvial flood plain deposits which are underlain by the Pleistocene Madhupur Clay deposits. It has tropical climate with less rainfall in summer and winter.

Gazipur: The sample is collected from agricultural site which is situated in Zirani bazar, Madhupur, Gazipur district. The GPS location of the sampling point where soil samples were collected was approximately 23˚59'45.5856''N and 90˚15'21.564''E. The area is categorized under red-brown general soil type (Figure 2). The Madhupur area is covered by Madhupur Clay Residuum of Pleistocene age, underlain by Plio-Pleistocene Dupitila Sandstone Formation and adjacent flood plains of the Jamuna and the old Brahmaputra river are characterized by the Holocene alluvium deposits. The climate of the Tract varies slightly from north to south, the northern reaches being much cooler in winter.

Malathion (57%) a commercial grade pesticide used mainly for pest control was collected from local pesticide shop from Dhaka, Bangladesh. The pesticide belongs to organophosphate class of pesticides.

The samples were collected from 8 - 10 cm depth using a sterile spatula and transferred to autoclaved Ziplock bag stored under room temperature. Figure 3 illustrates the flowchart of soil collection and processing for identification of phosphate solubilizing bacteria. Soil samples were air dried and crushed before sieving. 0.5 mm sieve was used for microbiological analysis and 2 mm sieve was used for determination of chemical properties of soil. To isolate bacteria, serial dilution plate technique was carried out (Greenberg et al. 1980) . When colonies appeared, they were counted and recorded and were purified through repeated plating (by streak plate method) (Cappuccino & Natalie, 2012) . These isolated colonies were identified by some morphological and biochemical tests such as shape, arrangement, colonies, form, margin, elevation, colour, Gram reaction,

Oxidase test, Catalase test, Oxygen requirement, Gelatin liquefaction, indole production test, methyl red and Voges-Proskauer test, citrate utilization, starch hydrolysis, Deamination of Phenylalanine Acid Production from D-Glucose, Gas Production from D-Glucose, Nitrate reduction and Triple sugar iron test. Furthermore, quantitative and qualitative tests were carried out by using National Botanical Research Institute’s phosphate growth medium (NBRIP) to identify phosphate solubilizing bacteria. By measuring the amount of phosphorous solubilized and appearance of halo zones, the identified bacteria were considered as phosphate solubilizing bacteria (Figure 4).

For determining the efficiency of isolated microorganism in degrading malathion pesticide National Botanical Research Institute’s phosphate growth medium (NBRIP) broth (modified)-contained glucose, 10 g: malathion [instead of Ca₃(PO₄)₂]; MgCl₂・6H₂O, 5 g; MgSO₄・7H₂O, 0.25 g; KCl, 0.2 g and (NH₄)₂SO₄, 0.1 g was used. The freshly prepared broths were carried out using autoclavable falcon (50 ml) containing 20 ml of medium and autoclaved at 121˚C for 15 minutes and incubated at 30˚C overnight. In the next day, broths were inoculated with isolated phosphate solubilizing bacterial colony (200 μl inoculum with approximately 2 - 3 × 10⁹ cfu∙ml⁻¹) in triplicate. Autoclaved uninoculated medium served as control. The broths were incubated for 2, 4, 6 and 10 days at different temperature (25˚C, 30˚C, 37˚C). The cultures were harvested by centrifugation at 10,000 rpm for 10 min, using centrifugal machine (Hettich Zentrifugen, Germany). Then the samples were filtered for phosphate determination (Figure 5). Inorganic phosphate in the culture supernatant was estimated following the vanadomolybdophosphoric yellow color method using a spectrophotometer as described by Jackson (Jackson, 1958) . The amount of inorganic phosphate solubilized in the supernatant was measured from the difference of sample and control media. The increase in inorganic phosphate in the supernatant was taken as the efficiency of microorganism in degrading malathion pesticide.

All soil conditions may not be suitable for growing all types of microorganisms. Vikram et al. (2007) reported a strong positive correlation between organic carbon content, available N and PSB population. Also, Ndung’u-Magiroi et al. (2012) showed a positive and highly significant correlation between PSB and phosphate-solubilizing microorganism (PSM) populations, organic C, exchangeable Ca and exchangeable Mg, while pH and extractable P did not correlate with the PSB population. Winogradsky detected saprophytic Bacillus species in soil, stating that the bacteria’s spores became active when an excess of easily decomposable organic matter became available or when the moisture content of the soil was high (Winogradsky, 1924) . Pseudomonas aeruginosa is found widely in nature, in soil and water and has few nutritional requirements and can adapt to conditions not tolerated by other organisms (Prince, 2012) . Therefore, the chemical parameters of soil samples were measured to determine favorable condition for the phosphate solubilizing Bactteria (Table 1). Among the 5 isolates, only 1 heterotrophic isolate was Gram-positive and rest were Gram-negative aerobic bacteria. Two bacteria named as Raoultella ornithinolytica and Pseudomonas pseudoalcaligenes isolated from agricultural soil of Sirajgonj, Pseudomonas pseudoalcaligenes and Bacillus cereus were isolated from moist soils of Tangail and lastly Pseudomonas aeruginosa and Bacillus subtilis were isolated from relatively moist agricultural field of Gazipur. Next, phosphate solubilizing bacteria were identified from isolated bacteria. Table 2 shows that out of the 5 isolates 4 showed phosphate solubilizing capacity. These were Raoultella ornithinolytica, Bacillus cereus, Pseudomonas aeruginosa and Bacillus subtilis. Bacillus cereus was the most efficient phosphate solubilizing bacteria (Figure 6).

In the present experiment, malathion was the sole phosphorous source for microbes in the broth. The amount of soluble phosphate released in the medium proved the degradation of malathion. Malathion degradation was observed at 25˚C, 30˚C and 37˚C which were presented in Tables 3-5. The phosphorous releasing capacity of these microbes varied with timings and incubation temperature.

At 25˚C and 30˚C, the amount of soluble phosphate in the medium increased on the fourth day of incubation and then decreased gradually. This indicated that organisms showed their highest degradation activity on the fourth day of incubation when incubation temperature was fixed at 25˚C and 30˚C [Figure 7(a) and Figure 7(b)]. In contrast, at 37˚C all organisms showed the highest degradation capacity on the second day of incubation except Bacillus Subtilis which degraded most of the pesticide on the sixth day [Figure 7(c)].

Temperature is one of the major environmental factors that influence biodegradation. Furthermore, bacterial metabolism may increase with an increase in temperature (Nadalian, et al., 2016) . In the experiment, temperature was found

to be a significant factor. It is reported that Bacillus species mainly grow in an optimum temperature of nearly 25˚C to 30˚C (Web, 1) , Pseudomonas aeruginosa at 25˚C to 37˚C (Prince, 2012) and Raoutella ornithinolytica at 37˚C (Chung, et al., 2016) . However, by measuring the amount of soluble phosphorous throughout the incubation period, it was demonstrated that for all organisms, 25˚C and 30˚C were the most favorable temperatures for degrading malathion [Figures 8(a)-(d)]. The release of the maximum amount of soluble

phosphate at 25˚C and 30˚C throughout the incubation period indicated the increase in metabolism activity due to increase in growth of bacterial cells. Moreover, Bacillus cereus followed by Pseudomonas aeruginosa was found to be the most efficient malathion degrading bacteria in this experimentby measuring total amount of phosphorous [Figure 8(a) and Figure 8(b)]. On the other hand, the release of the least amount of phosphorus by Raoultella ornithinolytica indicated that it was the least efficient in malathion degradation [Figure 8(d)].